The invention relates to a device for adjusting lower thread tension of a sewing machine, wherein an electomagnetic driving device is provided at a non-rotating part of the sewing machine, thereby rotating a first magnet via a transmission mechanism; a second magnet which is opposite to the first magnet with appropriate space, is rotatably provided with respect to a bobbin carrier; these magnets keep a certain one rotating relative position by means of the opposing magnet poles, thereby to transmit rotation of the first magnet to the second magnet; rotation of the second magnet actuates a tension adjusting mechanism which comprises a thread tension adjusting spring provided on the bobbin carrier, via the transmission mechanism so as to adjust the thread tension and controls the electromagnetic driving device in response to designation of the thread tension; and the tension of the lower thread may be adjusted, as the bobbin carrier is attached in the sewing machine.
In the prior art, the lower thread of the sewing machine is adjusted in tension by adjusting pressure of a thread tension adjusting spring provided in a bobbin carrier or a bobbin case. For doing adjustment, a sliding plate is opened and a bobbin case is taken out. Such work would be troublesome and could not be made as observing the sewing condition.
This invention is to provide a device which could adjust the thread tension by electric control, as attaching a mechanism concerned with the lower thread supply in the sewing machine.